Spin fluctuations in the light-induced high-spin state of cobalt valence tautomers

Abstract

We present a study of the static magnetic properties and spin dynamics in Cobalt valence tautomers (VT), molecules where a low-spin (LS) to high-spin (HS) crossover driven by an intramolecular electron transfer can be controlled by the temperature, by the external pressure or by light irradiation. In the investigated complex, a LS-Co(III) ion bound to a dinegative organic ligand can be reversibly converted into the HS-Co(II) bound to a mononegative one. By combining magnetization measurements with Nuclear Magnetic Resonance (NMR) and Muon Spin Relaxation (µSR), we have investigated the static magnetic properties and the spin dynamics as a function of the temperature. Moreover, the effect of the external pressure as well as of the infrared light irradiation have been explored through magnetometry and NMR measurements to determine the spin dynamics of the HS state. The photoinduced HS state, which can have a lifetime of several hours below 30 K, is characterized by spin dynamics in the MHz range, which persist at least down to 10 K. The application of an external pressure causes a progressive increase of the LS-HS crossover, which reaches room temperature for pressures around 10 kbar.

Magnetocaloric Effect in Carbon-Containing Compounds

“…The transition from a low spin to a high one caused by the intramolecular transfer of the electron was described in detail when the static magnetic properties and spin dynamics in zerosized valence cobalt tautomers was were studied [24]. It was shown that a spin-crossover can be controlled by temperature, external pressure or light radiation.…”

Putting the Squeeze on Valence Tautomerism in Cobalt-Dioxolene Complexes

“…While thermally-and photo-induced valence tautomerism has been explored extensively, less is known about pressure-induced valence tautomerism. [6][7][8][9] A family of complexes susceptible to valence tautomerism are [Co(Me n tpa)(dbdiox)] + (Me n tpa = tris(2pyridylmethyl)amine where n = 0-3, corresponds to successive methylation of the 6-position of the pyridine rings, dbdiox = 3,5-di-tert-butyl-1,2-dioxolene). 5-7,9−14 Previous analysis of valence tautomerism in this family of complexes by NMR and electronic spectroscopies, magnetic susceptibility and X-ray diffraction established that the charge distribution of the complex and its response to an external stimulus depends on the extent of methylation of the tpa ligand.…”

“…5-7,9−14 Previous analysis of valence tautomerism in this family of complexes by NMR and electronic spectroscopies, magnetic susceptibility and X-ray diffraction established that the charge distribution of the complex and its response to an external stimulus depends on the extent of methylation of the tpa ligand. 5,7,10 ]. (toluene) (denoted hereafter as Me 3 tpa) species were in the HS-Co II -SQ form under ambient conditions in the solid state.…”

“…11 For Me 2 tpa, Caracciolo et al (using magnetometry and NMR measurements) found that the application of pressure gradually increased the temperature of the HS-Co II -SQ to LS-Co III -Cat conversion until it reached room temperature at 1 GPa. 7 Pressure-induced valence tautomerism has not been probed at all for Me 3 tpa. While thermal valence tautomerism is driven by entropy, pressureinduced processes are driven by volumetric changes in the complex or crystal, generating distinct electronic and structural responses.…”

Putting the Squeeze on Valence Tautomerism in Cobalt-Dioxolene Complexes

“…While thermally-and photo-induced valence tautomerism has been explored extensively, less is known about pressure-induced valence tautomerism. [6][7][8][9]…”